The present invention relates to flow control valves. The invention more particularly concerns flow control valves which are adjustable in their ability to control various ranges of flow rates. The invention is especially directed at multiple flow range control valves whose ranges of flow rates may be changed without substantial replacement of the valves or valve parts.
The reciprocating element in a valve is commonly called a valve plug. By precisely controlling the position of the valve plug, the flow of fluid through the valve can be controlled. Thus, the parameters of a process or other operation may be maintained at a desired condition by precise modulating motion of the valve plug.
In general, mechanical regulating devices used for controlling the motion of a valve plug are composed of two devices a power device to produce the required force and motion, and a servomotor which controls the position of the power device. Valve positioners are a common form of servomotors.
Valve positioners are designed and constructed to respond to a command signal of some low level, and to provide an output of compressed air which causes movement of the power device. There is a feedback means between the power device and the positioner. The motion of the power device will cease when the feedback signal (or motion) has reached a level that corresponds to the command signal sent to the positioner.
The command signal to the positioner is the signal that results from one or more sensing or analyzing devices that are constantly monitoring process parameters in the fluids flowing through valves, piping or vessels. When there is a deviation from the desired parametric condition, the resultant signal to the positioner will cause the control valve to change the flow through the valve, thereby re-establishing the desired condition. The fluid conditions may have small deviations requiring small changes in flow, thus, requiring small precise movements of the power device.
In a process where a control valve is required, a computation that includes all process variables will typically disclose minimum, normal and maximum flow requirements of the valve. The flow requirement is expressed as Cv for each condition respectively. The term Cv is a unitless number that represents the flow coefficient of a valve. When a valve has a Cv of one, it will flow one U.S. gallon of water per minute when the pressure drop through the valve is one psi. From this basis, a guideline is established for selecting control valves. Thus, the Cv that is calculated as a requirement of a given process can be compared to published Cv values of various valves and an appropriate selection can be made.
The resulting control valve selection is normally based on the best possible valve to meet desired parametric conditions. When one or more process parameters change, a new computation may disclose that a valve with a higher or lower flow coefficient is required. In applications involving moderate to low flows, even a modest change in a parameter can create a situation where an existing control valve does not have the necessary range or flow capability to control the new process condition. For example, when the flow capacity of a valve must be increased or decreased, it may be necessary to change the internal components of the valve.
After a valve has been placed in service, it is an encumbrance to the user if he determines that the valve plug and orifice must be changed to provide the appropriate flow or control. Changes require isolating the valve from the process, removal from the process line, disassembly of the valve, selecting the appropriate plug and orifice, installation of the parts, re-assembly of the valve, calibration and pressure testing of the assembly, and reinstallation in the process line. In many chemical processes, this procedure can be costly and hazardous.
Control valves that use a contoured plug operating in conjunction with an orifice will have a rangeability that is typically 50:1. Rangeability is defined as the ratio of the maximum controllable flow to the minimum controllable flow. This may also be expressed as a ratio of the maximum Cv to the minimum Cv.